Indinavir Increases Midazolam N-Glucuronidation in Humans: Identification of an Alternate CYP3A Inhibitor Using an In Vitro to In Vivo Approach.

Midazolam is a widely used index substrate for assessing effects of xenobiotics on CYP3A activity. A previous study involving human hepatocytes showed the primary route of midazolam metabolism, 1'-hydroxylation, shifted to N-glucuronidation in the presence of the CYP3A inhibitor ketoconazole, which may lead to an overprediction of the magnitude of a xenobiotic-midazolam interaction. Because ketoconazole is no longer recommended as a clinical CYP3A inhibitor, indinavir was selected as an alternate CYP3A inhibitor to evaluate the contribution of the N-glucuronidation pathway to midazolam metabolism. The effects of indinavir on midazolam 1'-hydroxylation and N-glucuronidation were first characterized in human-derived in vitro systems. Compared with vehicle, indinavir (10 µM) inhibited midazolam 1'-hydroxylation by recombinant CYP3A4, human liver microsomes, and high-CYP3A activity cryopreserved human hepatocytes by ≥70%; the IC50 obtained with hepatocytes (2.7 µM) was within reported human unbound indinavir Cmax (≤5 µM). Midazolam N-glucuronidation in hepatocytes increased in the presence of indinavir in both a concentration-dependent (1-33 µM) and time-dependent (0-4 hours) manner (by up to 2.5-fold), prompting assessment in human volunteers (n = 8). As predicted by these in vitro data, indinavir was a strong inhibitor of the 1'-hydroxylation pathway, decreasing the 1'-hydroxymidazolam/midazolam area under the plasma concentration versus time curve (AUC)0-12h ratio by 80%. Although not statistically significant, the midazolam N-glucuronide/midazolam AUC0-12h ratio increased by 40%, suggesting a shift to the N-glucuronidation pathway. The amount of midazolam N-glucuronide recovered in urine increased 4-fold but remained <10% of the oral midazolam dose (2.5 mg). A powered clinical study would clarify whether N-glucuronidation should be considered when assessing the magnitude of a xenobiotic-midazolam interaction.

Assessment of a candidate marker constituent predictive of a dietary substance-drug interaction: case study with grapefruit juice and CYP3A4 drug substrates.

Dietary substances, including herbal products and citrus juices, can perpetrate interactions with conventional medications. Regulatory guidances for dietary substance-drug interaction assessment are lacking. This deficiency is due in part to challenges unique to dietary substances, a lack of requisite human-derived data, and limited jurisdiction. An in vitro-in vivo extrapolation (IVIVE) approach to help address some of these hurdles was evaluated using the exemplar dietary substance grapefruit juice (GFJ), the candidate marker constituent 6',7'-dihydroxybergamottin (DHB), and the purported victim drug loperamide. First, the GFJ-loperamide interaction was assessed in 16 healthy volunteers. Loperamide (16 mg) was administered with 240 ml of water or GFJ; plasma was collected from 0 to 72 hours. Relative to water, GFJ increased the geometric mean loperamide area under the plasma concentration-time curve (AUC) significantly (1.7-fold). Second, the mechanism-based inhibition kinetics for DHB were recovered using human intestinal microsomes and the index CYP3A4 reaction, loperamide N-desmethylation (KI [concentration needed to achieve one-half kinact], 5.0 ± 0.9 µM; kinact [maximum inactivation rate constant], 0.38 ± 0.02 minute(-1)). These parameters were incorporated into a mechanistic static model, which predicted a 1.6-fold increase in loperamide AUC. Third, the successful IVIVE prompted further application to 15 previously reported GFJ-drug interaction studies selected according to predefined criteria. Twelve of the interactions were predicted to within the 25% predefined criterion. Results suggest that DHB could be used to predict the CYP3A4-mediated effect of GFJ. This time- and cost-effective IVIVE approach could be applied to other dietary substance-drug interactions to help prioritize new and existing drugs for more advanced (dynamic) modeling and simulation and clinical assessment.

A modified grapefruit juice eliminates two compound classes as major mediators of the grapefruit juice-fexofenadine interaction: an in vitro-in vivo "connect".

The grapefruit juice (GFJ)-fexofenadine interaction involves inhibition of intestinal organic anion transporting polypeptide (OATP)-mediated uptake. Only naringin has been shown clinically to inhibit intestinal OATP; other constituents have not been evaluated. The effects of a modified GFJ devoid of furanocoumarins (~99%) and polymethoxyflavones (~90%) on fexofenadine disposition were compared to effects of the original juice. Extracts of both juices inhibited estrone 3-sulfate and fexofenadine uptake by similar extents in OATP-transfected cells (~50% and ~25%, respectively). Healthy volunteers (n = 18) were administered fexofenadine (120 mg) with water, GFJ, or modified GFJ (240 mL) by randomized, three-way crossover design. Compared to water, both juices decreased fexofenadine geometric mean AUC and C(max) by ~25% (P ≤ .008 and P ≤ .011, respectively), with no effect on terminal half-life (P = .11). Similar effects by both juices on fexofenadine pharmacokinetics indicate furanocoumarins and polymethoxyflavones are not major mediators of the GFJ-fexofenadine interaction.